1. Field of the Invention
The present invention relates to an information processing apparatus such as a distance measuring apparatus for automatic focusing of a camera, a copying machine, a facsimile apparatus, an image reader or a video camera, and more particularly a photoelectric converting device to be employed as an image sensor in such apparatus.
2. Related Background Art
The U.S. Pat. No. 4,686,554 granted to inventors Ohmi and Tanaka discloses a charge-accumulating photoelectric converting device of high sensitivity and a low noise level, in which the emitter of a bipolar transistor is connected to an output circuit including a capacitative load. In recent years, however, there are requirements for a higher resolving power, a higher sensitivity and a lower noise level for the photoelectric converting device.
Considered particularly troublesome are noises. The noises in a photoelectric converting device are principally fixed pattern noises and random noises.
Since the fixed pattern noises (FPN) can be compensated for by an external correction circuit, the basic requirement lies in the reduction of the random noises (RN). Stated differently, the reduction of the random noises leads to a higher performance of the sensor.
FIG. 1 is a schematic cross-sectional view of a unit cell of a conventional photoelectric converting device, wherein are shown an n-Si substrate 2', an n.sup.- -epitaxial area 2, a p-base area 3, an emitter area 4, and an emitter electrode 5. Also a p-MOS transistor is constituted by a p-area 62, the p-base area 3 and a gate electrode 61. In this photoelectric converting device, among the carriers generated by the incident light, the holes are accumulated in the base, and a signal corresponding to thus accumulated carriers is released from the emitter. Subsequently the p-MOS transistor forms an inversion layer under the gate electrode, whereby the base area 3 is maintained at the same potential as that of the drain area 62 to which a predetermined potential is supplied through the drain electrode 63, and the carriers are thus extracted from the base.
The various experiments of the present inventors have revealed that a cause of the random noises in the photoelectric converting device of the charge accmulating type lies in the incomplete transfer of photogenerated signal carriers in the resetting operation.
Another cause is the noise generation in the functioning of the transistor at the output signal readout.